In wet-laid nonwovens, it is necessary to bond together the relatively short fibers which constitute the nonwoven in order for the resulting web to have any significant strength. Generally, liquid binders and/or binder fibers are utilized for this purpose. In the case of liquid binders, a polymer solution or dispersion (e.g. latex) is applied to the nonwoven web and subsequently dried. While significant strength can be achieved through this method, there are issues which it can create. The first of these is that the liquid binder requires additional process steps in its application. Specifically, the binder solution/dispersion must be applied in a manner to yield a uniform distribution of the binder polymer in the nonwoven sheet. Wet-laid nonwovens can often include fibers with wide-ranging wettability to such liquid materials (e.g. cellulosic versus synthetic fibers) such that uniform application of the liquid binder can prove a challenge. Also, once applied, the liquid binder must be dried in order for the nonwoven manufacture to be complete. There is not only an energy expenditure required by this process (high heat of vaporization for water) but non-uniform binder levels which may be present at the nonwoven surface can result in sticking of the web to high temperature drying cans which are used in this process
Binder fibers, on the other hand, are fiber materials which can be readily combined with other fibers in a wet-laid furnish but which differ somewhat from typical “structural” fibers in that they can be thermally-activated or softened at a temperature which is lower than the softening temperature of the other fibers present in the nonwoven. Current binder fibers suffer from the fact that they can typically be rather large (approximately 10-20 microns) compared to other fibrous materials present in the sheet. This larger size can result in rather significant adverse changes to the pore size/porosity of the nonwoven media. In addition, monocomponent binder fibers (e.g. polyvinyl alcohol) at these relatively large diameters have low surface-to-volume ratios which can result in the melted polymer flowing and filling nonwoven pores much in the way that liquid binders do.
As a partial solution to this problem, core-sheath binder fibers are often employed. In a core-sheath binder fiber, the sheath polymer has a melting point that is lower (typically by >20° C.) than that of the core polymer. The result is that at temperatures above the sheath melting point but below the core melting point, the sheath bonds to other fibers present in the nonwoven web while the core allows the core-sheath binder fiber to maintain a largely fibrous state, such that, unlike the aforementioned polyvinyl alcohol fibers, the pores of the nonwoven are less likely to be blocked. However, core-sheath binder fibers are still rather large fibers which can significantly increase the average pore size of a nonwoven web.
There is a need in the paper and nonwoven industry for a binder fiber which is (1) sufficiently small not to adversely increase the pore size/porosity of a nonwoven (particularly at utilization rates which would impart high strength), and (2) capable of maintaining a fibrous morphology after thermally bonding with other fibers in the nonwoven web (i.e. after it melts).